10.6.2 Time Zone Leap Second Support

Leap second values are returned with a time part that ends with
:59:59. This means that a function such as
NOW() can return the same value
for two or three consecutive seconds during the leap second. It
remains true that literal temporal values having a time part
that ends with :59:60 or
:59:61 are considered invalid.

If it is necessary to search for
TIMESTAMP values one second
before the leap second, anomalous results may be obtained if you
use a comparison with 'YYYY-MM-DD hh:mm:ss'
values. The following example demonstrates this. It changes the
local time zone to UTC so there is no difference between
internal values (which are in UTC) and displayed values (which
have time zone correction applied).